For example, gas sensors, which contain a catalyst, which is heated to a predetermined temperature, as a result of which the combustible gases are catalytically burnt on the surface of the sensor while consuming part of the oxygen present in the gas being measured and raise the temperature in the process, are used to detect combustible/toxic gases. The rise in the sensor temperature occurring during the combustion reaction is analyzed as a measured signal for the concentration of the gas in the air mixture to be analyzed.
To carry out the measurement, a catalytically active sensor element and a passive sensor as a compensator are usually arranged in one half of a bridge, the passive sensor being used to compensate the ambient temperature effect. The detuning of the bridge is an indicator of the catalytic conversion of the combustible gas component at the catalytically active sensor element.
To suppress inflammation of the combustible gas, at least the catalytically active sensor element is accommodated in a sensor housing, which is covered, for example, with a porous, gas-permeable sintered material as a flame trap.
A catalytically active gas sensor of this type is known from EP 94 863 A1. The catalytically active sensor element is located in a sensor housing, which housing is delimited by a porous, gas-permeable sintered material. Two metal pins, which contact the sensor element, are led through a glass pane on the underside of the sensor housing to the outside. The sensor element is surrounded by zeolite material in order to reduce the energy consumption due to the insulating action and adsorption properties of that material and to prolong the service life.
The prior-art glass seal for the metal pins is not suitable for use in explosion-proof sensor housings encapsulated in a pressure-proof manner. Such sensor housings must be dimensioned such that they withstand 1.5 to 4 times the pressure that can build up in the interior of the sensor housing in case of an explosion. Even a pressure resistance of up to and above 400 bar is required in pertinent standards. A flat pane of glass material, i.e., a glass pane with a small thickness to diameter ratio, which is weakened, moreover, by the integration of a plurality of metal pins, is destroyed by cracking in case of exposure to the pressure emanating during a gas explosion.
Pressure-proof encapsulated sensors which comprise a metal housing, which is embedded in a plastic material, are known as well. Certain minimum casting thicknesses must be maintained in such housing designs and compliance with certain standard requirements must be demonstrated. A sensor in a plastic housing appears, for example, from WO 2004/048955 A1.
A gas sensor, in which the metal pins contacting the sensor element are fused each individually into separate glass inserts in the bottom plate of the sensor housing, is known from DE 10 2005 020 131 B3. Even though the prior-art gas sensor has a high pressure resistance due to the glass inserts arranged individually, the manufacturing process is relatively complicated. A separate hole must be prepared for each metal pin in the bottom plate. The metal pins must be placed very exactly centrally in their holes because of the small hole diameters necessary to achieve the pressure resistance in order to prevent an electric short-circuit between the metal pin and the bottom plate. In addition, specially selected glass/metal pairs and/or sealing oxide layers are necessary because of the thermal expansion and a minimum thickness of the bottom plate and glass inserts is necessary for the purpose of mechanical stability.